Sculpt high detail with custom brush over low poly terrain for Unity
I would model a low poly Terrain...UV unwrap.....copy the terrain name it same name as first just add "HI_poly" to name (so it is easy to tell between the 2) add a multi res to the Hi poly...sculpt out your hi poly...(if you have an issue with it crashing you are running out of ram when it bakes the normal map)

stay under a million verts on the hi poly(hopefully that will keep it from crashing...that's what I had to do when I had just 4 gigs of ram)

once you are happy with your hi poly sculpt...

take your Lo poly terrain paint just a basic color on an image save the image...add a materiel to the lo poly and add that basic color image to that materiel....now first select the hi poly mesh, then shift select the lo poly mesh and bake an ambient occlusion map....then bake your normal map....(make sure you have selected to active in the bake panel) save the images I use _AO _NORM...ECT ECT

now paint out your lo poly color map in as much detail as you can....use that image for color take the images into GIMP and create our color map by overlaying the AO....you can also make your spec map...de-saturate the color image... invert the colors and adjust to likeness....

hope this helps.

oh and save often.